1. Field of the Invention
This invention relates to the composition of passivation and fuse protective layers used in integrated circuits (ICs), and in particular, to a specified oxynitride composition for a fuse protective or passivation layer that avoids hydrogen diffusion into polysilicon structures such as resistors.
2.Description of Related Art
Many integrated circuits (ICs), such as Static Random Access Memory (SRAM), employ resistors that must have a specified resistance value after fabrication. An example is U.S. Pat. No. 5,172,211 to Godinho et al. entitled "High Resistance Polysilicon Load Resistor" incorporated here in its entirety by reference. Another example is U.S. Pat. No. 5,166,771 to Godinho et al. entitled "Self-Aligned Contact and Interconnect Structure" also incorporated here in its entirety by reference.
In the manufacture of ICs having resistors, some of the processing steps performed after resistor formation may undesirably alter the resistances. Some of these steps take place after formation of a fuse protective layer or a passivation layer.
Fuse protective layers protect underlying IC elements while the IC is being tested and repaired. One example of a fuse protective layer is used in an SRAM IC. It is well known in the art to provide in an SRAM redundant memory cells or redundant columns of memory cells, and to use the redundant cells to repair the SRAM if some of the cells in the SRAM are defective. To permit replacement of cells, IC repair fuses or anti-fuses are included in the SRAM for disconnecting defective cells and replacing the defective cells with redundant cells. During manufacturing, the SRAM is tested, and if defective cells are found, appropriate repair fuses are blown and defective cells are replaced.
Typically, to prevent damaging the SRAM during test and repair, the SRAM is covered with a fuse protective layer. The fuse protective layer defines openings to make electrical connections therethrough, and windows that permit passage of a laser beam to blow underlying fuses. The fuse protective layer otherwise protects the remainder of the SRAM from being scratched or contaminated during test and repair. Materials used as fuse protective layers include silicon dioxide, doped silicate glass, silicon nitride, and oxynitride, and combinations of the above.
After the test and repair, a passivation layer is formed over the fuse protective layer to cover up the fuse openings and to further protect the IC from scratches, moisture, and chemical contaminants. Typically, a silicon nitride passivation layer is deposited using chemical vapor deposition.
ICs also undergo thermal processes, such as alloying to anneal charges, after the formation of a fuse protective layer or after the formation of a passivation layer. During formation of fuse protective and passivation layers and during subsequent alloying of the IC, the resistances of IC resistors may change. In an SRAM the change in resistance of a load resistor undesirably changes the voltage retention capabilities of a cell, and may make some cells inoperable. The production yield of SRAM ICs is thereby decreased, increasing unit costs.
Consequently, materials and methods are needed for forming fuse protective and passivation layers that do not alter the resistance of underlying resistors during formation of the layer or when the IC is subjected to thermal processes.